Curiosity Sees Unearthly Moondance In Martian Skies

Mars’ moon Deimos is occulted by Phobos on Aug. 1 as seen by Curiosity

What fun to live on a planet with TWO moons. Imagine stepping out into the Martian night to watch the moons Phobos and Deimos chase each other across the sky. NASA’s Curiosity rover did just that on Aug. 1 when mission control pointed its mast camera at the pair of tiny moons and snapped 41 photos as the larger and closer Phobos passed directly in front of little Deimos. In real time the “eclipse” took 55 seconds; the movie compresses that to 11. Even on Mars it was a marvelous night for a moondance.

With only one moon here on Earth, we miss out on the pleasures of dual moon gazing. The only thing that might come close is watching a cargo ship like the recent HTV-4 catch up and dock with the International Space Station.

Phobos orbits closer to Mars than Deimos and therefore completes a revolution around the planet more quickly, regularly overtaking its brother. The photos are the very first ever taken from Mars of an eclipse of one moon by the other.

Comparison showing how big the moons of Mars appear to be, as seen from its surface, in relation to the size that our moon appears to be seen from the Earth’s surface. Credit: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.

A 100mm telephoto lens was used to make the images which clearly show some of the larger craters on Phobos.

Both moons are tiny compared to our own. Deimos’ diameter is 7.5 miles (12 km) and Phobos 14 miles (22 km). It takes me longer to drive to work than cross the length of Deimos.

Even though Phobos is only about twice the size of Deimos, it appears much larger from the surface because it orbits much closer to the Red Planet – 3,700 miles (6,000 km) vs.12,400 miles (20,000 km).

Orbiting above the Martian equator and so close to the surface, Phobos can’t be seen from Mars’ polar regions. Its great speed also means it overtakes the planet’s rotation rate, rising in the west and setting in the east during the Martian night. Here on Earth, the moon moves in the same west to east direction but much more slowly, so that the faster-rotating Earth shuttles it from east to west during the night.

Phobos and Deimos up close as photographed by spacecraft. NASA scientists are studying the recent Curiosity images to determine precise orbits for the two moons as well as to gain a better understanding of the interior of Mars. Credit: NASA

Phobos’ tight orbit will ultimately lead to its demise. Its gravity induces tidal bulges in the crust of Mars which lag behind the fast-orbiting moon, causing it to gradually slow down and drop closer to the planet’s surface. In 50-100 million years Phobos will spiral in close enough for Mars’ gravity to break it to pieces. Deimos alone will remain to dimly light the Martian night.

7 Responses

Great question Carol. Since the moons are about Venus’ brightness or brighter and the exposures were made to record details on Phobos, other virtually all other stars would be invisible. To get a shot of Venus here on Earth by itself, you can record it in under a second. That amount of time wouldn’t show any other stars except maybe Vega, Sirius and the like.